1. Field of the Invention
The present invention relates to the composition and utilization of transparent polymers which are suitable in particular for the manufacture of rigid-type contact lenses.
2. Description of the Prior Art
It is known that, for this type of application, polymers must essentially be permeable to the gaseous oxygen of the atmosphere since absence of oxygen in the vicinity of the cornea prohibits wearing of the lenses over long periods. In the case of polymers of acrylic esters (usually of the methacrylic type) which are in particularly frequent use as contact lens materials, it has often been proposed to modify them by copolymerization with organic siloxanes in order to improve their permeability to oxygen. However, since the introduction of siloxanes produces acrylic silicones, it has the disadvantage of reducing other properties which are required of contact lenses, such as transparency, shock resistance, breaking strength, dimensional stability and durability.
Up to the present time, the research effort to solve these difficulties have essentially remained directed towards the choice of special highly branched polysiloxanes carrying unsaturated end groups which permit copolymerization with the acrylic esters of the basic composition, to which are usually added crosslinking agents constituted by polyfunctional unsaturated monomers. Transparent polymers for rigid contact lenses are described in particular in French patents No. 2,255,320, No. 2,417,782, No. 2,517,834, No. 2,533,933. In all cases, the silicone-type constituent is an acrylic (or methacrylic) ester of an organosiloxane. In the molecules therefore considered here, all the silicon atoms of the ##STR1## chains are saturated by carbon groups such as methyl or phenyl and the property of polymerization is conferred by acrylic (or methacrylic) groups fixed on methylene radicals at the end of the chain.
These polymers have failed to provide a satisfactory solution to the above-mentioned problem of oxygen permeability of contact lenses. Moreover, the choice of suitable constituents appears complex and polymerization of these acrylic esters involves reactions of the radical type which, in practice, prohibit the manufacture of lenses by casting directly in their final shape.
In order to avoid the various disadvantages of the prior art techniques, the present invention proposes to make use of silicones or silanes of a different type in which the reactivity is due to silyl ##STR2## groupings and to copolymerize them with crosslinking in a rigid matrix with a constituent having ethylenic double bonds.
In point of fact, polymerization reactions of this type, by hydrosilylation of olefinic bonds, have already been used, but in other fields of application of polymers involving compositions which lead to global properties other than those which are sought for rigid contact lenses. Thus French patent No. 2,536,752 describes compositions prepared from diene polymers having both dangling double bonds and silyl end groups, possibly with addition of disilylsilanes as coupling agents, for the sole purpose of giving flexibility to cross-linked polymers which serve as binding agents in adhesives or coatings. Other patents such as U.S. Pat. No. 3,284,406, U.S. Pat. No. 3,436,366 and U.S. Pat. No. 4,077,943 contemplate reactions of the same type within a silicone elastomer between vinyl end groups of an organopolysiloxane and the silyl groups of an organohydrogenopolysiloxane. Finally, the addition reaction of a compound having a vinyl, acrylic or methacrylic double bond on the silyl groups of a partially silane organopolysiloxane (therefore an organohydrogenopolysiloxane) has sometimes been employed, as in European patent No. 0,033,754, for grafting hydrophilic molecules at the surface of contact lenses of silicone elastomer.